Microscale Transport Phenomena in Materials Processing

Microscale transport mechanisms play a critical role in the thermal processing of materials because changes in the structure and characteristics of the material largely occur at these or smaller length scales. The heat transfer and fluid flow considerations determine the properties of the final product, such as in a crystal drawn from silicon melt or a gel from the chemical conversion of a biopolymer. Also, a wide variety of material fabrication processes, such as the manufacture of optical glass fiber for telecommunications, fabrication of thin films by chemical vapor deposition, and surface coating, involve microscale length scales due to the requirements on the devices and applications for which they are intended. For example, hollow fibers, which are used for sensors and power delivery, typically need fairly precise microscale wall thicknesses and hole diameters for satisfactory operation. The basic transport mechanisms underlying these processes are discussed in this review paper. The importance of material characterization in accurate modeling and experimentation is brought out, along with the coupling between the process and the resulting properties such as uniformity, concentricity, and diameter. Of particular interest are thermally induced defects and other imperfections that may arise due to the transport phenomena involved at these microscale levels. Additional aspects such as surface tension, stability, and free surface characteristics that affect the material processing at microscale dimensions are also discussed. Some of the important methods to treat these problems and challenges are presented. Characteristic numerical and experimental results are discussed for a few important areas. The implications of such results in improving practical systems and processes, including enhanced process feasibility and product quality, are also discussed.